1. Field of the Invention
This invention relates to a process and an apparatus for absorbing hydrogen sulphide, more specifically a process and an apparatus in black-liquor evaporation for the selective removal, by liquid absorption, of hydrogen sulphide from a gas containing hydrogen sulphide as well as carbon dioxide.
2. Description of Conventional Art
Hydrogen sulphide is absorbed chiefly in the form of hydrogen-sulphide ions (HS.sup.-) but also in the form of sulphide ions (S.sup.2-). In the following, the total content of these ions will be collectively referred to as the "total sulphide content" or, more briefly, as the "sulphide content".
It is well-known that hydrogen sulphide can be removed from hydrogen-sulphide-containing gases by being absorbed in an alkaline aqueous solution, such as sodium hydroxide, or by using ethanolamine, such as monoethanolamine and diethanolamine. The absorption method may, for instance, be used for producing hydrogen sulphide in pure form, and optionally further processing to sulphur in a Claus process. If the gas contains carbon dioxide in addition to hydrogen sulphide, the carbon dioxide will also be absorbed in the alkaline solution. Carbon dioxide has approximately the same solubility in water as hydrogen sulphide, and the carbon dioxide will therefore compete with the hydrogen sulphide for being absorbed in the solution. Hydrogen sulphide and carbon dioxide are absorbed in an alkaline aqueous solution of e.g. sodium hydroxide in accordance with the formulae below. EQU H.sub.2 S+OH.sup.-.fwdarw.HS.sup.- +H.sub.2 O (1) EQU H.sub.2 S+2OH.sup.-.fwdarw.S.sup.2- +2H.sub.2 O (2) EQU CO.sub.2 +OH.sup.-.fwdarw.HCO.sub.3.sup.- (3) EQU CO.sub.2 +2OH.sup.-.fwdarw.CO.sub.3.sup.2- +H.sub.2 O (4)
The selectivity for hydrogen sulphide, i.e. the ratio of mole of absorbed hydrogen sulphide to mole of absorbed (hydrogen sulphide+carbon dioxide), is directly proportional to the hydrogen sulphide and carbon dioxide contents of the gas. Thus, the competition on the part of carbon dioxide is especially pronounced when the gas contains more carbon dioxide than hydrogen sulphide, as is mostly the case in actual practice. If a gas contains, say, 1% by volume of hydrogen sulphide and 10% by volume of carbon dioxide and efforts are made to absorb the hydrogen sulphide in a sodium hydroxide solution, the selectivity for hydrogen sulphide is merely 10%, i.e. 90% of the gas absorbed is made up of carbon dioxide, which means that as much as 90% of the sodium hydroxide is spent in absorbing carbon dioxide.
In an effort to remedy the above inconvenience in the absorption of hydrogen sulphide from gases containing hydrogen sulphide as well as carbon dioxide, methods for selective absorption of hydrogen sulphide have been developed. For instance, efforts have been made to selectively absorb hydrogen sulphide in solutions of strong oxidising agents, such as potassium permanganate, sodium dichromate or ferric salts. In other selective methods, use is made of alkaline solutions, such as sodium carbonate or potassium carbonate solutions, the operational conditions being carefully adjusted in the absorption. More detailed information about this prior-art technique is found in an article by C. Oloman, F. E. Murray and J. B. Risk, entitled "The Selective Absorption of Hydrogen Sulphide from Stack Gas", Pulp and Paper Magazine of Canada, Dec. 5, 1969, p. 69 ff, as well in an article by E. Bendall, R. C. Aiken and F. Mandas, entitled "Selective Absorption of H.sub.2 S from Larger Quantities of CO.sub.2 by Absorption and Reaction in Fine Sprays", AICHE Journal (Vol. 29, No. 1), January 1983, p. 66 ff.
An instance of the prior art is described in U.S. Pat. No. 3,554,859, which relates to a method for recovering sulphur from furnace gases generated in the combustion of e.g. black liquor. The combustion gases, which contain hydrogen sulphide and carbon dioxide, are contacted with a gas containing molecular oxygen and with an aqueous alkaline solution containing sodium ions, e.g. in the form of sodium hydroxide and sodium carbonate or sodium carbonate and sodium hydrogencarbonate. Thus, hydrogen sulphide is absorbed from the gas and oxidised to thiosulphate. The absorption is rendered even more effective by an addition of ferric oxide, the sulphide concentration of the solution being thus maintained at a very low level, i.e. the sulphide content of the solution should be minimised.
By using a carbonate solution, such as a sodium carbonate solution, instead of a hydroxide solution, such as a sodium hydroxide solution, the selectivity for the absorption of hydrogen sulphide can be augmented to about 30-50%. The reactions taking place during such absorption can generally be rendered as follows. ##STR1##
When the absorption solution is a carbonate solution, the hydrogen sulphide is absorbed almost instantaneously, whereas the carbon dioxide reacts only slowly with the carbonate ions to form hydrogen carbonate ions. Owing to the high content of hydrogen carbonate generated when using a carbonate solution as absorption medium, there is the additional advantage of a "counterpressure" (equilibrium pressure) to the absorption of carbon dioxide, as appears from the equilibrium formula (6) above.
A problem that arises when using a carbonate solution as absorption medium is that only a relatively low sulphide content can be achieved in the solution, owing to the reduction of the absorption capacity caused by the formation of hydrogen carbonate ions. Thus, it is extremely difficult to attain sulphide contents exceeding about 0.30 mole/l. As a result, prior-art methods for selective absorption of hydrogen sulphide by means of an absorption medium in the form of a carbonate solution have not met with much success, despite the great demand for such a method in the many fields where hydrogen-sulphide-containing and carbon-dioxide-containing gases are generated. Examples of such fields of application are petroleum refinement, coal-gas production and, in particular, the combustion of black liquor carried out in the sulphate pulp industry.
When recovering chemicals in the sulphate industry in accordance with the conventional Tomlinson process, the black liquor is burnt in a soda recovery unit, resulting in the generation of steam and the formation of a melt chiefly consisting of sodium carbonate and sodium sulphide. The melt is then dissolved in water and causticised, so that the sodium carbonate is converted to sodium hydroxide and white liquor is obtained, which may then again be used for digesting wood. For many reasons, including the risk of an explosion when a tube in the soda recovery unit bursts, efforts have in recent years been made to develop new processes for the combustion of black liquor, in which the black liquor is pyrolysed under reducing conditions and in which no melt is formed.
Such processes can be collectively referred to as "black-liquor evaporation", and one instance thereof is the so-called SCA-Billerud process (E. Horntvedt and J. Gomy, Paper Trade Journal 158 (1974):16, pp 32-34). In this process, the black liquor is pyrolysed in a reactor under such temperature conditions that dust, which chiefly consists of sodium carbonate and carbon, and a combustible gas, which inter alia contains sulphur compounds, are formed. Another instance of black-liquor evaporation is given in U.S. Pat. No. 4,872,950, which relates to a method for thermal decomposition of black liquor with concurrent supply of oxygen in an amount short of the stoichiometrically required amount, at a pressure above 10 bar, and at such a temperature that no melt is formed. The evaporation results in the formation of a solid phase, which chiefly consists of sodium carbonate, and a gaseous phase, which chiefly consists of hydrogen sulphide, carbon monoxide, carbon dioxide, hydrogen, water vapour, and methane.
EP 459,962 is concerned with the cleaning of process gas in black-liquor evaporation. In the cleaning, sulphur compounds and sodium compounds are removed from the gas by the contact with alkaline solutions containing hydrogen sulphide ions and hydroxide ions. The cleaning involves at least two stages. In the first stage, the gas is passed through a venturi nozzle along with an alkaline solution, so that melted aerosol drops of black liquor in the gas are absorbed by the solution. Then, the gas is contacted with a solution containing hydroxide ions and hydrogen sulphide ions in a molar ratio exceeding 4:1. The high molar ratio of hydroxide ions to sulphide ions results in the absorption solution having a low sulphide concentration. The alkaline solutions used in the absorption, such as white liquor or wash liquor, have high pH values of about 13-14, resulting in poor selectivity for the absorption of hydrogen sulphide. Furthermore, the absorption solution employed has a low carbonate content, and it is specifically indicated that green liquor, which has a high carbonate content, cannot be used as washing solution.
In order to be able to recover the chemicals used in black-liquor evaporation, i.e. the evaporation of black liquor by combustion in a reducing atmosphere, and produce from these chemicals white liquor to be used in the manufacture of pulp, it is necessary that the hydrogen sulphide can be removed from the generated gas. Since the gas also contains carbon dioxide, the latter will compete with the hydrogen sulphide in the liquid absorption, and since the gas has a low content of hydrogen sulphide (about 0.5-2%) while the carbon dioxide content is some 20 times higher (about 10-20%), conventional liquid absorption results in unsatisfactory recovery of hydrogen sulphide.